A crankcase lubricant that performs adequately in one engine at given operating conditions does not necessarily perform adequately when used in a different engine or under different conditions. While theoretically, lubricants could be designed for each possible combination of engine and service condition, such a strategy would be impracticable because many different types of engines exist and the engines are used under different conditions. Accordingly, lubricants that perform well in different types of engines and across a broad spectrum of conditions (e.g. fuel type, operating load, and temperature) are desired. Design of crankcase lubricants is further complicated in that the concentrated mixture of chemicals added to lubricating oil basestocks to impart desirable properties should perform well over a broad range of different quality basestocks. Meeting these requirements has been extremely difficult because the formulations are complicated, tests to ascertain whether a lubricant performs well are extremely expensive and time consuming, and collecting field test data is difficult since variables cannot be controlled sufficiently.
Ever more stringent regulation of vehicle emissions make the task even more challenging. Most recently, in North America the maximum permitted level of sulfur present in over the highway diesel fuel has been lowered to 0.05 wt %. A new API category defined as API CG-4 addresses the performance of heavy duty lubricants with low sulfur fuels. At the same time fuel sulfur levels for off-highway equipment may have a higher level. Furthermore, in some geographical regions such as Latin America, fuel sulfur levels remain high for all applications. Lubricants for use in heavy duty diesel engines therefore need to perform acceptably across a range of sulfur fuel levels.
To meet this need the American Petroleum Institute ("API") issues certification licenses for lubricants that pass a panel of tests designed to verify a lubricant's performance in a variety of engines operated at conditions that have been associated with lubrication problems. In addition to the requirements for API licensing, manufacturers of heavy duty diesel engines periodically have required lubricants to pass additional tests before the lubricant can be approved for use with that manufacturer's engines. Lubricants that meet all the certification requirements of heavy duty diesel engine manufacturers and all the requirements for the highest level of the American Petroleum Institute's oil service classifications for both gasoline fueled engines and heavy duty diesel fueled engines are often referred to as universal oils.
Among the many tests that heavy duty diesel lubricants have been required to pass are the Caterpillar 1G2 and the more recent Caterpillar 1M-PC, 1K, and 1N tests. Acceptability of an oil is based on control of oil consumption and piston deposits (top groove fill, top land heavy carbon, and weighted deposits). Stuck piston rings or distress of the piston, its rings, or its liner will also disqualify an oil. The Caterpillar 1N requires a low sulfur fuel (0.05 wt %) while the 1K uses a fuel with traditional sulfur levels (0.4 wt %).
The Mack T-7 test and its successor the Mack T-8 are part of a panel used to determine acceptability of oils for engines manufactured by Mack Truck Company. The Mack T-7 (the Mack Truck Technical Services Standard Test Procedure N0. 5GT 57 entitled "Mack T-7: Diesel Engine Oil Viscosity Evaluation", dated Aug. 31, 1984) tests soot related viscosity increase in diesel engines.
The Mack T-8 (the Mack Truck Technical Services Standard Test Procedure entitled "Mack T-8: Diesel Engine Oil Viscosity Evaluation", dated October 1993) evolved because the fuel injection timing in some newer engines has been retarded to enable the engines to meet emission requirements. At the same time fuels have been reformulated to have lower sulfur content altering physical and chemical properties of the soot. Some engines designed to run on low sulfur fuel with retarded fuel injection have experienced excessively high soot related viscosity increases, excessively high filter pressure drops, and excessive sludge deposits. The Mack T-8 test runs for 250 hours with an engine operating at 1,800 RPM with an applied load 1010-1031 lb.-ft (1369.4-1397.8 newton-meters). Throughout the test, the soot levels, the differential pressure across the oil filter, and kinematic viscosity of the test fluid are measured. The measured viscosities and soot levels are used to interpolate a viscosity at 3.8 wt % soot level. An oil passes the test if that viscosity differs from the lowest viscosity measured in the test by 11.5 cSt or less. If two tests are run the two results when averaged must be 12.5 cSt or less. If three tests are run the three results when averaged must be less than 13 cSt. An additional requirement includes control of filter pressure differential. The Mack T-8 is much more severe than the Mack T-7 test and requires a higher dispersancy level in the fluid.
Another test required of heavy duty diesel lubricants is the CRC L-38 (ASTM D5119). That test is run on a single cylinder laboratory gasoline engine. It is designed to test a lubricant's ability to prevent corrosion of a bearing made from copper and lead, and to prevent sludge and varnish formation.
Heavy duty diesel engines must also perform satisfactorily in off road conditions. The John Deere Company, a manufacturer of farm equipment, is concerned about high temperature performance of lubricants because oil coolers sometimes become covered with mud. The John Deere 6466A High Temperature Engine Oil Test Procedure (JDQ-78) tests high temperature thermal oxidative oil thickening in a heavy duty diesel engine.
Among the many tests required to meet the API SH classification (and its predecessor the API SG classification) are the Sequence IID (ASTM STP 315h part 1), Sequence IIIE (ASTM D553), and Sequence VE (ASTM D5302). The Seq. IID (ASTM STP 315h part 1) monitors an oil's ability to inhibit rust. It is intended to simulate cold winter conditions for short trip driving when condensation on the valve cover creates a corrosive environment. The Seq. IIIE (ASTM D553) measures high temperature oil thickening, sludge and varnish deposits, and engine wear. The Seq. VE (ASTM D5302) measures the lubricant's ability to prevent deposits and wear encountered during low-temperature, light duty operating conditions. Primary rating factors include measurement of sludge, varnish, and camshaft wear in the engine.
Another factor complicating design of lubricants is the well known problem that an additive, or combination of additives, that improves performance in one respect may make the cost of the lubricant too high or may adversely affect performance in another respect.
Ripple U.S. Pat. No. 5,202,036 describes a formulation designed to pass the Caterpillar 1G2, the Mack T-7, and the CRC L-38 tests. U.S. Pat. No. 5,202,036 uses two parameters to indicate the amount (TBN) and source (metal ratio) of basicity of a given material. Total Base Number, "TBN", is an industry standard used to correlate the basicity of any material to that of potassium hydroxide. The value is reported as mg KOH and is measured according to ASTM D2896. "Metal ratio" is a calculated value that relates the total amount of metal present to number of equivalents of metal required to saturate the anion of the organic acid. If the metal ratio is 1, the amount of metal present is the amount required to saturate the anion of the organic acid. If the metal ratio is greater than 1, metal in excess of that required to saturate the anion is present. The term "overbased" may be used to describe any metal salt of an organic acid having a metal ratio greater than 1 though typically overbased sulfonates will be used at metal ratios in excess of 2.
U.S. Pat. No. 5,202,036, describes a lubricant that has a TBN in the range of 6 to about 15 and has a specifically defined dispersant and an alkali or alkaline earth metal salt of an organic acid having a metal ratio greater than at least about 2 wherein the specifically defined dispersant provides from 0.5 to 1.5 TBN and the metal salt component includes a magnesium salt or salts such that the magnesium salts or salts contribute no more than about 30% of the TBN of the composition. The patent does not address tests required for gasoline fueled engines or the Mack T-8 test.
U.S. Pat. No. 4,941,984 to Chamberlin describes a lubricant intended for use with spark ignited engines fueled by gasoline, alcohol, or mixtures of both. It requires a metal detergent that is either a basic magnesium salt of an organic acid or a basic mixture of alkaline earth metal salts of one or more organic acid wherein at least 50% of the metal is magnesium together with a metal (other than magnesium or calcium) salt of either a substituted succinic acid acylated polyamine or a hydrocarbon substituted aromatic carboxylic acid containing at least one hydroxyl group attached to the aromatic ring. Chamberlin defines "basic" when applied to the magnesium salts as follows: "The basic magnesium salt and other basic alkaline earth metal salts . . . are referred to as basic salts because they contain an excess of magnesium or other alkaline earth metal cation. Generally, the basic or overbased salts will have a metal ratio of about 2 to about 30 or 40." He asserts that his lubricants having high levels of basic (i.e. metal ratio .gtoreq.2) magnesium salts when used in alcohol fueled engines or mixed alcohol/gasoline fueled engines minimize corrosive wear and pre-ignition problems associated with alcohol fueled engines. Chamberlin further asserts that his lubricant can be formulated to qualify for API "SG" classification. The Chamberlin patent is not concerned with the unique problems associated with soot related viscosity increase or piston deposits formed in diesel engines.
Another patent addressing a way to use Group II metal hydrocarbyl sulfonates to achieve high TBN while recognizing that they are deleterious in other respects is EP 277,729 to Rollin. Rollin's formulation includes zinc dithiophosphate having both primary (1.degree.) and secondary (2.degree.) character such that the ratio of primary:secondary is from about 1:1 to about 5:1, a succinimide dispersant; and a TBN in finished oil of at least 8. He states the succinimide is necessary to pass friction tests and the upper limit on the amount of succinimide present does not matter to performance but is determined solely by cost. While Rollin reports a significant number of engine tests, no Mack T-7 results are shown. Nor does he show any test results where the metal salt of a dihydrocarbyl dithiophosphoric acid made from secondary alcohol exceeds the amount made from primary alcohol.
Additional patents relating to the use of zinc dithiophosphates made from specific secondary alcohols and used with specific dispersants are U.S. Pat. Nos. 4,904,401, 4, 957,649, and 4,981,602 all to Ripple.
Despite all the work that has gone before, a need remains for lubricants that perform extremely well in diesel tests that use low sulfur fuel, including the Mack T-8 and the Caterpillar 1N, without compromising performance in the older tests and that deliver the requirements for the highest API classification for lubricants intended to be used in gasoline fueled engines--superior resistance to oxidation, rust, and wear.